System and method for filtering output in mass flow controllers and mass flow meters

ABSTRACT

Embodiments of the present invention can include the steps of (i) filtering the indicated flow to derive an intermediate filtered indicated flow, (ii) deriving a weighted first derivative of the intermediate filtered indicated flow, and (iii) outputting a filtered indicated flow comprising the sum of the intermediate filtered indicated flow and the weighted first derivative of the intermediate filtered indicated flow. The indicated flow can be filtered using a FIR, an IIR a running average or other filtering scheme.

RELATED INFORMATION

This application is a continuation of, and claims a benefit of priorityunder 35 U.S.C. 120 of the filing date of U.S. patent application Ser.No. 10/133,110 by inventor Faisal Tariq entitled “System and Method forFiltering Output in Mass Flow Controllers and Mass Flow Meters” filed onApr. 26, 2002 now U.S. Pat. No. 6,865,520, which further claims priorityunder 35 USC 119(e) to provisional patent application No. 60/286,934,entitled “MFC & MFM Output Filter,” by Faisal Tariq filed Apr. 27, 2001,each of which is hereby expressly incorporated by reference for allpurposes.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to mass flow controllers andmass flow meters and more particularly to a output filter for mass flowcontrollers and mass flow meters.

BACKGROUND OF THE INVENTION

As the silicon wafers used by semiconductor manufacturers increase insize, more accurate control of gas flow through the manufacturingequipment has become more critical for the precise fabrication ofcircuits on the wafers. Mass flow controllers and mass flow meters aretypically used to control and monitor gas flow through a semiconductormanufacturing tool. In the manufacturing process, if the controller ormonitor detects a fluctuation in gas pressure that is outside predefinedoperating parameters for a particular manufacturing tool, an alarm isusually triggered to shut down the tool. This is often costly tomanufacturers as it reduces throughput and because the wafer batch beingprocessed when the tool is shut down is usually ruined. Even if thebatch is not ruined or damaged, however, additional wafer batches cannotbe processed until the tool is brought back online. This is even moreannoying and inefficient when the shutdown of the manufacturing tool isdue to a false alarm (i.e. a non-critical event).

In this regard, conventional mass flow controllers and mass flow metersare deficient because they often produce false alarms due to noise ortransient pressure spikes that do impact the manufacturing process byexceeding the manufacturing tolerances. In a mass flow controller, forexample, a thermal flow sensor reads the flow of gas to a manufacturingtool. The conventional thermal flow sensor, however, has severallimitations, one of these being that the time constant used by thethermal flow sensor to read gas flow through the system is much longerthan the desired time necessary to control the flow. In other words, bythe time the thermal flow sensor reads an event it is too late to reactto the event by controlling the gas flow or shutting down the system.One method of accelerating or predicting gas flow faster than providedby the conventional thermal flow sensor is to derive a weighted firstderivative of the signal generated by the thermal flow sensor and addthe weighted derivative to the signal, producing an indicated flow. Theindicated flow is then compared to a set point for the manufacturingtool. If there is an error (e.g., if the indicated flow does not matchthe set point) a gas flow valve will typically be throttled so that theindicated flow matches the set point, yielding zero error, or if theerror is greater than a predetermined level, the tool is shut down.

FIG. 1 is a diagrammatic representation illustrating a prior art system100 for regulating gas flow. In system 100, the actual gas flow 105 ismonitored by thermal flow sensor 110. Thermal flow sensor 110 samplesthe actual flow 105 and outputs signal 115 representing the actual gasflow 105. To accelerate or predict the flow faster, a derivativecontroller 120 (“D-controller” 120), derives a weighted first derivativeof signal 117 by multiplying a predetermined gain times the firstderivative of signal 115 to produce derivative signal 117. Derivativesignal 117 is then added to signal 115 via summer 119 to produceindicated flow 125.

Indicated flow 125 can be compared to a set point for the manufacturingtool involved (e.g., at a comparator 127). If the indicated flow doesnot match the set point (e.g., if an error is detected) a gas flow valvecurrent or actuator current (signal 137) is generated to throttle valve130, thereby regulating the actual flow 105 and yielding a zero error.The throttling of valve 130 is achieved via a proportional and integralcontroller 135 (“P&I controller” 135). Thus, while thermal flow sensor110 operates on a time scale longer than desired for the control ofactual flow 105, the output of thermal flow sensor 115 is manipulated toachieve faster response, thus providing a means to control the gas flow,if a critical event is encountered, in a timely manner avoiding damageto the current work product.

While an improvement in monitoring and controlling of gas flows, theseprior art systems still have several shortcomings. Thermal flow sensor110 is located at only one location in actual flow path 105 and can onlydetect local flow. Thus, thermal flow sensor 110 may detect localinstabilities in actual flow 105 that are not representative of the flowas a whole. For example, if thermal flow sensor 110 is located at anarea of local turbulence (e.g., near a bend in the gas flow path), itmay pick up local instabilities that are not representative of what isactually occurring downstream at the fabrication chamber. Furthermore,the sensor may itself cause eddy currents at its mouth, thereby causingthermal flow sensor 110 to produce an inaccurate or noisy signal 115.The prior art system is further deficient in that derivative signal 117typically enhances any noise present in signal 115. Thus, indicated flow125 includes enhanced noise that is often not representative of actualflow 105.

When indicated flow 125 is compared to the set point for a tool(typically after indicated flow 125 reaches steady state), if themismatch between indicated flow 125 and the set point is greater thanthe threshold valve, an alarm is generated triggering a shut down of thetool, typically ruining the batch of wafers upon which work is currentlybeing performed. This mismatch often is not due to levels of noise inactual flow 105, but may be caused by the enhanced noise present inderivative signal 117. To compensate for noise in indicated flow 125,P&I controller 135 includes filtering capabilities. However, thefiltering capabilities of conventional mass flow meters are limitedbecause they can not adequately filter noise that spans over a broadfrequency range.

Furthermore, these prior art systems are limited because they generallydo not handle pressure spikes well. If there is a brief pressure spikein actual flow 105, the spike, which would be represented in indicatedflow 125, can cause an alarm condition (e.g., can cause the tool to shutdown) even if the spike would not affect the production process byexceeding its tolerances. Thus, the prior art systems cause unnecessarydowntime due to noise or transient pressure differences.

Therefore, a need exists for a filter that is independent of the noisefrequency, is less affected by transient spikes and does not compromisethe response time of the tool to which it is being applied.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstancesand can be characterized by one aspect as an output filter thatsubstantially eliminates or reduces disadvantages and problemsassociated with conventional filters. More particularly, embodiments ofthe present invention provide a method for filtering an indicated flow.Additional aspects and advantages of the invention will be set forth inpart in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theaspects and advantages of the invention will be realized and attained bymeans of the elements and combinations particularly pointed out in theappended claims.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, the presentinvention can be characterized according to one aspect of the inventionas comprising the steps of (i) determining if the indicated flow iswithin an allowable deviation from a baseline, (ii) if the indicatedflow is within the allowable deviation, outputting the indicated flow asthe filtered indicated flow, and (iii) if the indicated flow is notwithin the allowable deviation, outputting a value that is withinallowable deviation as the filtered indicated flow. For example, theoutput filter can output the last sample of the indicator flow that waswithin the allowable deviation. Embodiments of the present invention canalso include determining if the indicated flow has exceeded a buffer. Ifthe indicated flow has not exceeded the buffer, one embodiment of thepresent invention can continue to output the value that is within theallowable deviation. If, however, the indicated flow has exceeded thebuffer, the output filter can engage a timer having a clock limit. Ifthe indicated flow does not return to within the allowable deviationbefore the timer reaches a clock limit, the embodiment of the presentinvention can output the indicated flow, which remains outside of thebuffer, as the filtered indicated flow.

Another aspect of the present invention can be characterized asincluding the steps of (i) filtering the indicated flow to derive anintermediate filtered indicated flow, (ii) deriving a weighted firstderivative of the intermediate filtered indicated flow, and (iii)outputting a filtered indicated flow comprising the sum of theintermediate filtered indicated flow and the weighted first derivativeof the intermediate filtered indicated flow.

Embodiments of the present invention provide an important technicaladvantage with respect to previous filtering techniques by eliminatingnoise across a range of frequencies while outputting persistent changesin an indicated flow.

Embodiments of the present invention provide another important technicaladvantage by removing transient spikes from a filtered indicated flow.

Embodiments of the present invention provide yet another importanttechnical advantage by significantly reducing the likelihood that afalse alarm condition will occur due to noise or transient spikes in anindicated flow.

Embodiments of the present invention provide yet another importanttechnical advantage by eliminating noise and transient spikes from anindicated flow without reducing the response time of the system to whichthe present invention is being applied.

It is to be understood that both the foregoing general description andthe following detailed description are explanatory only and are notrestrictive of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings which are incorporated in and constitute apart of this specification, illustrate several embodiments of theinvention and together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a diagrammatic representation illustrating a prior art systemfor regulating gas flow;

FIG. 2 is a schematic representation of one embodiment of a mass flowcontroller that can utilize embodiments of a output filter according tothe present invention;

FIG. 3 is a graphical representation of one embodiment of the process offiltering an indicated flow in an embodiment of a output filteraccording to the present invention;

FIG. 4 illustrates a flow chart for the same embodiment of the outputfilter according to the present invention that is implemented with amass flow controller;

FIG. 5 is a diagrammatic representation of a mass flow meterimplementing one embodiment of a output filter according to the presentinvention; and

FIG. 6 provides a flow versus time graph for one embodiment of afiltering process occurring at the same embodiment of a output filterfor a mass flow meter.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to exemplary embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

One embodiment of the present invention can include a mass flowcontroller capable of comparing an indicated flow to a baseline (e.g., aset point) and determining if the indicated flow is within an allowabledeviation of the baseline. If the indicated flow is within the allowabledeviation from the baseline, the mass flow controller can output theindicated flow as a filtered indicated flow. Otherwise, the mass flowcontroller can output an value within the allowable deviation frombaseline as the filtered indicated flow.

Another embodiment of the present invention can include a mass flowmeter capable of generating a filtered indicated flow by (i) filteringan indicated flow to derive an intermediate filtered indicated flow;(ii) deriving a weighted first derivative of the intermediate filteredindicated flow; and (iii) outputting a filtered indicated flowcomprising the sum of the intermediate filtered indicated flow and theweighted first derivative of the intermediate filtered indicated flow.

Embodiments of the present invention provide an output filter capable offiltering noise in an indicated flow across a range of frequencies, andembodiments of the present invention provide the capability to filtertransient spikes in an indicated flow. Thus, noise and pressure spikespresent in the indicated flow can be filtered so that they do notgenerate false alarms and/or unnecessary gas flow manipulations. For thepurposes of this application, “indicated flow” will refer torepresentation of actual flow output by various flow measuring and/orregulating devices. The indicated flow is capable of including thedirect output of a flow sensor or a manipulated output. For example, theindicated flow is capable of including the output of a thermal flowsensor summed with a weighted derivative of the thermal flow sensor'soutput. While the indicated flow typically includes the weighted firstderivative of a flow sensor's output, it is also capable of includingthe second, third, etc. derivatives, or other mathematical manipulationsin whole or in part. Furthermore, the indicated flow can include anyrepresentation of an actual flow that is used to monitor, regulate orcontrol the actual flow.

FIG. 2 is a schematic representation of one embodiment of a mass flowcontroller 200 that is capable of utilizing embodiments of a outputfilter according to the present invention. The mass flow controllerincludes digital and/or analog components and measures the actual flow205 via a thermal flow sensor 210. Thermal flow sensor 210 outputs andelectrical signal 215 representing actual flow 205. As would beunderstood by one of ordinary skill in the art, since the time constantused by thermal flow sensor 210 may be longer than the time scaledesired for controlling actual flow 205, signal 215 is manipulated (e.g.via a Derivative Controller and Summer) to achieve a faster responsetime. Accordingly, D-controller 220 in one embodiment of mass flowcontroller 200, generates a first derivative mass flow signal andmultiplies it by a gain to produce derivative signal 217. Summer 219 canadd derivative signal 217, which can include enhanced noise introducedby D-controller 220, to signal 215 to produce indicated flow 225. Itshould be noted that, in one embodiment of the present invention,indicated flow 225 can be essentially the same as the indicated flow 125produced by prior art mass flow controllers.

D-controller 220 can, as would be understood by one of ordinary skill inthe art, enhance the response time of mass flow controller 220. However,this may be done at the expense of enhancing noise in the output ofthermal flow sensor 210. Thus, derivative signal 217 and, hence,indicated flow 225 can each include noise that does not normally show upin actual flow 205 (i.e. can include the enhanced noise added byD-controller 220). Noise (due to pressure spikes and sudden flowchanges) in a indicated flow 225 can cause a tool to generate falsealarms, thereby causing unnecessary shut downs of production lines.Output filter 230, however, is capable of filtering indicated flow 225to remove noise or spikes from indicated flow 225, thereby producing afiltered indicated flow 235 that generates fewer false alarms andshutdowns. It should be noted that for the purposes of this applicationthe term “filtered indicated flow” means the output of output filter 230whether or not the output has been manipulated by output filter 230.

In addition to being filtered by output filter 230, indicated flow 225can be compared (at comparator 227) to a set point (e.g., such as adesired point for a manufacturing tool or piece of manufacturingequipment) to determine if the actual flow 205 should be adjusted (e.g.,throttled or increased). As would be understood by those of ordinaryskill in the art, one consideration in adjusting actual flow 205 are thetolerances required by the particular manufacturing process in which thepresent invention is implemented. If the actual flow 205 should beadjusted according to the comparison of indicated flow 225 to the setpoint, P&I controller 240 opens or closes gas flow valve 245 to throttleor increase actual flow 205. Additionally, the filtered indicated flow235 can be compared to a set point for the manufacturing tool todetermine if conditions deviate appreciably from the set point for asufficient time period, warranting shutting down the tool. If, forexample, the filtered indicated flow 235 falls outside of a specifiedoperating range, an alarm signal, is generated to shut valve 245.However, because output filter 230 can remove noise and transient spikesfrom indicated flow 225, the manufacturing tool less likely to shut downdue to these types of false alarm conditions.

It should be noted that, in the mass flow controller employing oneembodiment of an output filter according to the present invention, theoutput filter is not in the feedback loop. It should be further notedthat the configuration of mass flow controllers illustrated in FIG. 2 isexemplary only and other configurations of mass flow controllers can beemployed and not deviate from the techniques of the present invention.One example of a mass flow controller which could include an embodimentof an output filter according to the present invention can be found inU.S. patent application Ser. No. 09/350,744, “System and Method ofOperation of a Digital Mass Flow Controller,” which is hereby fullyincorporated by reference. Furthermore, embodiments of the presentinvention can be implemented in micro-processor based mass flowcontrollers and analog based mass flow controllers. The use of a digitalsignal processor (“DSP”) platform, in particular, can makeimplementation more streamlined.

FIG. 3 is a graphical representation of one embodiment for processingand filtering an indicated flow of a output filter according to thepresent invention. Flow vs. time graph 300 represents the filtering ofindicated flow 225 of one embodiment of an output filter 230 accordingto the present invention. On graph 300, a user-defined set point (“SP”)is represented by line 310. As would be understood by one of ordinaryskill in the art, a set point can be defined by the user, for example,when a manufacturing tool is powered up or when the user wishes toredefine the set point to account for new operating conditions. The setpoint establishes a baseline value against which output filter 230 cancompare data points taken from the indicated flow 225. The output filteris configurable to wait a predetermined amount of time (e.g., t seconds,represented by line 315) after a particular event (e.g., a power up orchange in set point) to allow the indicated flow 225 to achieve anapproximately steady state. It should be understood that t can be anadjustable value and can be user defined. During this delay time “t”,the indicated flow 225 can be passed as the filtered indicated flow 235.By not filtering the indicated flow during the delay time, the overallresponse of the output filter 230 can be increased and a user can gaininsight into any overshoots of the set point or other trends that mayoccur prior to indicated flow 225 reaching an approximately steadystate.

In the embodiment shown in FIG. 3, after t seconds (line 315), outputfilter 230 can begin monitoring indicated flow 225 to determine when theindicated flow 225 is within an allowable deviation from the baseline(e.g., in this embodiment, plus or minus one percent of the SP,represented by lines 320 and 325, respectively). After t seconds (i.e.,the delay time expires), and once the indicated flow 225 is determinedto be within the allowable deviation (e.g., plus or minus one percent ofSP in this case), represented by point 330, the output filter 230engages to filter noise and/or transient spikes. Filtering of indicatedflow 225 can occur in real-time (i.e., as fast as computer andcommunication technology will allow). In one embodiment of the presentinvention, output filter 230 may not begin filtering at exactly the timewhen indicated flow 225 crosses into the plus or minus one percentrange. Instead, output filter can sample indicated flow 225 and beginfiltering indicated flow 225 when the first sample of indicated flow 225falls within the plus or minus one percent band (assuming t seconds haveexpired).

Once output filter 230 is engaged, output filter 230 can sampleindicated flow 225 to determine if indicated flow 225 falls outside ofan allowable deviation. Again, in the case shown in FIG. 3, the outputfilter 230 samples indicated flow 225 to determine if indicated flow 225deviates from the set point by more than plus or minus one percent(i.e., the allowable deviation delineated by lines 320 and 325,respectively). When a sample taken by the output filter 230 indicatesthat indicated flow 225 is within the plus or minus one percent band (inthe example of FIG. 3), output filter 230 can output the sample as thefiltered indicated flow 235. In other words, output filter 230 can passindicated flow 225 as filtered indicated flow 235. In a DSP mass flowcontroller, this can include passing the most recent sample of theindicated flow to the output port of the DSP. Alternatively, if thesample of the indicated flow 225 falls outside of the allowabledeviation, output filter 230 can output a value from within theallowable deviation as filtered indicated flow 225. For example, whenoutput filter 230 determines, at point 343, that indicated flow 225 hascrossed outside of the plus or minus one percent band (i.e., is aboveline 320, in this example) output filter 230 can output the last samplethat fell within the allowable deviation (or any other value from withinthe allowable deviation), indicated by point 342, as the filteredindicated flow 235 (represented by line 355). Thus, when indicated flow225 falls outside of the allowable deviation from the setpoint, outputfilter 230 can output, at least for awhile, a filtered indicated flow235 that falls within the allowable deviation. In one embodiment of thepresent invention, output filter 230 can output the value of the lastsample of indicated flow 225 that fell within the allowable deviation asthe filtered indicated flow.

In one embodiment of the present invention, the output filter is capableof outputting a value within the allowable deviation (e.g., plus orminus 1%) so long as indicated flow 225 does not exceed a buffer.However, when a sample (e.g., taken at point 345) indicates thatindicated flow 225 exceeds the buffer, in this case plus or minus threepercent (represented by lines 335 and 340, respectively), output filter230 can engage a timer (not shown). The timer runs until the timer hasreached its clock limit (e.g., a predefined run time) or indicated flow225 returns to within the buffer (e.g., plus or minus 3% or some otherpredetermined range) whichever comes first.

Once the timer is started at least three events can affect the valuesthat output filter 230 passes as the filtered indicated flow: indicatedflow 225 can return to within the allowable deviation before the timerruns out, the timer can run out before indicated flow 225 returns towithin either the buffer or the allowable deviation, or indicated flow225 can return to within the buffer, but not the allowable deviation,before the timer reaches its clock limit. Further discussing each ofthese cases, if indicated flow 225 returns to within the allowabledeviation before the timer reaches the clock limit, output filter 230can resume outputting indicated flow 225 as the filtered indicated flow235. If, however, indicated flow 225 returns to within the buffer, butnot the allowable deviation, before the clock limit is reached, as shownby point 346, output filter 230 can reset the timer and can continue topass a good value (e.g., a value within the allowable deviation) as thefiltered indicated flow 235. Otherwise, if the timer runs out beforeindicated flow 225 returns to within either the buffer or the allowabledeviation, output filter 230 can output the indicated flow 225 as thefiltered indicated flow 235. In this last case, however, the filteredindicated flow will exceed the buffer.

By calibrating the timer to have a sufficiently long clock limit,transient spikes and noise are effectively filtered out of the filteredindicated flow 235, and, by defining a sufficiently short clock limit,persistent changes in indicated flow can be included in the filteredindicated flow. In one embodiment of the present invention, for example,the clock limit can be on the order of 100 milliseconds Thus, the usercan define a run time which filters out noise and transient spikes whichoccur at higher frequencies, but does not filter out important changesin indicated flow 225, which typically occur at lower frequencies.

As further examples, FIG. 3 illustrates that output filter 230 samplesthe indicated flow 225 just before it passes outside of the plus orminus one percent band. Therefore, the output filter outputs the valueof the last good sample (e.g., point 360) as the filtered indicated flow235. Coincidentally, in this example, the output filter 230 samples theindicated flow 225 just as the indicated flow 225 crosses back into theplus or minus one percent band (at point 365), and thus returns tooutputting indicated flow 225 as the filtered indicated flow. It shouldbe noted that in the spike between point 360 and point 365, theindicated flow 225 did not cross outside of the buffer and therefore,the timer did not start running to measure the transience of the spike.

At point 370, the output filter 230 again takes a sample of indicatedflow 225 which falls outside of the plus or minus one percent band(e.g., lines 320 and 325) and therefore output filter 230 beginscommunicating the value of the last good sample (e.g., taken at point375) as the filtered indicated flow (represented by line 380). When thesample at point 385 indicates that the indicated flow 225 has returnedto being within the plus or minus one percent band, the output filter230 resumes outputting the indicated flow 225 as the filtered indicatedflow (e.g., lines 320 and 325). Again, the filtered indicated flow 225did not cross outside of the plus or minus three percent buffer (e.g.,lines 335 and 340) between point 375 and 385, and thus output filter 230did not begin the timer.

At point 390 a sample of indicated flow 225 falls outside of theallowable deviation (e.g. lines 320 and 325) and again the output filter230 outputs in the last good sample (taken at point 392) as the filteredindicated flow 235 (represented by line 394). At point 395, the outputfilter 230 takes a sample indicating that the indicated flow 225 hascrossed outside of the plus or minus three percent buffer. Therefore,output filter 230 starts the timer while continuing to output the lastgood sample (taken at point 392) as the filtered indicated flow 235.When the timer has run for predetermined amount of time (point 396)without a sample of the indicated flow 305 returning to within the plusor minus three percent buffer, output filter 230 can resume outputtingthe indicated flow 225 as the filtered indicated flow (represented atline 398). As can be noted from FIG. 3, the indicated flow 225 nowremains outside the buffer. Because the indicated flow 225 remainsoutside the plus or minus three percent buffer (e.g. lines 335 and 340)for a significant amount of time (e.g., longer than the clock limit)between points 395 and 396, the output filter 230 can determine that theindicated flow 225 is not representing noise or transient spikes in theactual flow, but is, instead, indicating a material change in the actualflow. Further, because the filtered indicated flow 235 will exhibit,after point 396, a deviation from the set point that falls outside ofthe second predefined range an alarm signal can be generated or otherresponse be implemented.

In another embodiment of the present invention, rather than abruptlyswitching from outputting the filtered indicated flow, represented byline 390 flow, to outputting the indicated flow 225 as the filteredindicated flow, output filter 230 can gradually bring the filteredindicated flow to match the indicated flow 225.

It should be understood that while FIG. 3 describes a specificembodiment of the present invention having a plus or minus one percentallowable deviation and a plus or minus three percent buffer, FIG. 3 isnot limiting and other embodiments of the present invention areavailable. For example, both the allowable deviation from baseline andthe buffer can be user definable and can comprise percent deviationsfrom the baseline, absolute deviations or other statistical deviationsknown in the art. Additionally, in one embodiment of the presentinvention, only an allowable deviation, but not a buffer, can bedefined. Thus, for example, an embodiment of the output filter of thepresent invention can be configured to start the timer when a sample ofthe indicated flow falls outside of the plus or minus one percent (orotherwise defined) allowable deviation. However, the buffer can beprovided so that very small offsets (e.g., offsets just outside of theplus or minus one percent range) do not start the clock and are notrepresented in the filtered indicated flow. Furthermore, it should beunderstood that the allowable deviation and the buffer can beasymmetrical about the baseline. As just one example, allowabledeviation could be plus one percent and minus two percent, etc.

In the foregoing examples, output filter 230 compared the indicated flowto a baseline defined by a set point. However, in other embodiments ofthe present invention, a user can arbitrarily define the baseline.Additionally, a user can define the delay time “t” and the timer's clocklimit. It should be noted that graph 300 is not to scale and t secondscan be on the order of multiple seconds while the timer clock limit(e.g., the time difference between point 395 and point 396) can be onthe order of milliseconds.

FIG. 4 illustrates a flow chart for one embodiment of the output filteraccording to the present invention that is implemented with a mass flowcontroller. At step 410, the output filter determines if thepredetermined time delay (e.g., t seconds in FIG. 3) has passed. If thetime delay has not passed, the output filter can pass indicated flow 225as filtered indicated flow 235 (e.g. step 415), otherwise control passesto step 430, wherein the output filter can determine if indicated flow225 has come within an allowable deviation of a baseline. In the exampleof FIG. 4, output filter 230, at step 430, will determine if theindicated flow is within plus or minus one percent of a set point. If atstep 430, the indicated flow signal is within the allowable deviation,the output filter will begin filtering the indicated flow 225 at step435. Otherwise, the indicated flow will be passed as the filteredindicated flow until the indicated flow comes within the allowabledeviation (step 415). By allowing for a delay of t seconds anddetermining if the indicated flow is within the allowable deviation ofthe baseline before filtering, the indicated flow 225 can achieve anapproximately steady state before filtering begins.

If t seconds have passed (step 410) and indicated flow 225 has comewithin the allowable deviation (step 435), output filter 230, at step440, can determine whether or not the indicated flow has exceeded theallowable deviation from a baseline. If not, the output filter continuesto pass the indicated flow as the filtered indicated flow (step 415).If, on the other hand, the indicated flow is outside of the allowabledeviation, output filter 230 can pass a good value of the indicated flowas the filtered indicated flow (step 450), and control passes to step455. In one embodiment of the present, output filter 230, at step 450,can pass the last good sample of the indicated flow as the filteredindicated flow. In other embodiments of the present invention, outputfilter 230 can pass any value within the allowable deviation as thefiltered indicated flow if it is determined that the indicated flowexceeds the allowable deviation.

If, at step 440, the indicated flow has exceeded the allowabledeviation, output filter 230, at step 455, can determine if theindicated flow also exceeds a buffer. In the example of FIG. 4, outputfilter 230, at step 455 will determine if the value of indicated flowexceeds the baseline by more than three percent. If the indicated flowdoes not exceed the buffer, output filter 230 can return to step 440 anddetermine if the indicated flow (or the next sample of the indicatedflow) exceeds the allowable deviation (e.g., plus or minus one percentof the set point). If at step 455, output filter 230 determines,conversely, that the indicated flow exceeds the buffer, output filter230 can engage a timer (step 457). Output filter 230 can then determineif the timer reaches its clock limit (i.e., a predetermined run time)before the indicated flow returns to within the buffer (step 460). Ifthe indicated flow returns to within the buffer before the timer reachesits clock limit, output filter 230 can return to step 440. Otherwise,the output filter can pass the indicated flow, which has remainedoutside of the buffer for at least the run time of the clock (indicatinga continuous or persistent change in actual flow), as the filteredindicated flow. Again, output filter 230, in one embodiment of thepresent invention, can bring the filtered indicated flow to graduallymeet the value for the indicated flow, thereby avoiding passing sharptransitions in values in the filtered indicated flow. The processdescribed in conjunction with FIG. 4 can, in one embodiment of thepresent invention, be implemented as a looped subroutine to an existingprogram (e.g., a main program) already utilized by the mass flowcontroller, though the process can be implemented in any other mannerknown to those in the art (as a stand alone program, for example).

In summary, an embodiment of the present invention is capable ofoutputting the indicated flow as the filtered indicated flow when theindicated flow is within a predefined range about the set point (e.g.,when the indicated flow is within an allowable deviation of a baseline).However, if the indicated flow crosses outside of that predefined range,the last value of the indicated flow that was within the predefinedrange can be output as the filtered indicated flow. Furthermore, if theindicated flow deviates from the set point by a large enough amount(e.g., is outside of the buffer), for a long enough time (e.g., greaterthan the clock limit), the indicated flow can be output as the filteredindicated flow. Hence, high frequency noise or short duration spikes inthe indicated flow can be represented by the last good value of theindicated flow, while persistent deviations from the set point in theindicated flow will also be represented in the filtered indicated flow.As noise and transient spikes are removed from the indicated flow whilepersistent deviations in the indicated flow are represented in thefiltered indicated flow, the filtered indicated flow provides a betterunderstanding of changes in the actual flow that are likely tomaterially affect the system without effecting the response time of themass flow meter. Because noise and transient spikes can be removed fromthe indicated flow while persistent deviations can be represented,embodiments of the output filter according to the present invention canoutput a filtered indicated flow which is less likely to cause falsealarms. Furthermore, because manufacturing tools are less likely to shutdown due to false alarms, the present invention can decrease downtimeand prevent the loss wafer batches, leading to significant cost savings.

In an alternative embodiment, the output filter of the present inventiondoes not have to actually produce a filtered indicated flow. In thisembodiment, the present invention would not pass any signal until theoutput filter determines that the timer has reached its clock limitprior to the indicated flow returning to within the buffer. If theoutput filter determines that the indicated flow has not returned tobeing within the buffer prior to the timer reaching its clock limit, theoutput filter could then begin passing the original indicated flow asthe filtered indicated flow. In other words, the filtered indicated flowcan be the same as the indicated flow.

In addition to being used with mass flow controllers, embodiments ofoutput filters according to the present invention can be implementedwith mass flow meters. FIG. 5 is a schematic representation of a massflow meter 500 implementing one embodiment of a output filter accordingto the present invention. At mass flow meter 500, a thermal flow sensor510 is capable of measuring the actual flow 515 of a fluid through asystem and outputting and electrical signal 520 to represent actual flow515. D-controller 525 can determine the weighted amount of the firstderivative of signal 520 by multiplying a gain times the firstderivative of signal 520. D-controller 525 can output the weightedamount of the first derivative of signal 520 as derivative signal 527,which can then be added to signal 520 to establish an indicated flow530. Again, D-controller 525, which can determine the weighted amount ofthe first derivative of signal 520 in order to increase the responsetime of the system, can enhance noise found in signal 520. As will bedescribed in greater detail below, output filter 540 can then (i) filterthe indicated flow 530 through running average, FIR, IIR or otherfiltering scheme as would be understood by those of ordinary skill inthe art to derive an intermediate filtered indicated flow, (ii) derive aderivative of the intermediate filtered indicated flow, (iii) multiplythe derivative by a gain, which can be the same as or different than thegain applied by D-controller 525, and (iv) add the intermediate filteredindicated flow to the derivative of the intermediate filtered indicatedflow multiplied by the gain to derive a filtered indicated flow 620. Inone embodiment of the present invention, output filter 540 can beimplemented using a DSP platform.

FIG. 6 provides a flow versus time graph for one embodiment of afiltering process of embodiment of a output filter for a mass flow meterof the present invention. Again, in one embodiment of the presentinvention, this process can be carried out in real-time. The outputfilter can receive an indicated flow, represented by line 530. Aspreviously stated, the indicated flow 530 can contain enhanced noiseproduced by D-controller 525. In order to compensate for the enhancementof noise and any transient spikes that may appear in indicated flow 530,the output filter can filter the indicated flow 530 to produce afiltered indicated flow 620. To derive filtered indicated flow 620, oneembodiment of the output filter 540 can first apply standard filteringtechniques, such as running average, IIR, FIR or other techniques thatwould be understood by those of ordinary skill in the art, to indicatedflow 530 to produce an intermediate filtered indicated flow 630. Theoutput filter can then determine a derivative (e.g., a first derivative)of the intermediate filtered indicated flow 530 and multiply thederivative by a gain (“K”), the results of which are represented byweighted derivative line 640. Output filter 540 can then add theintermediate filtered indicated flow 630 and the weighted derivative ofthe intermediate filtered indicated flow 640 to produce the filteredindicated flow signal 620. If implemented using a DSP (or othermicro-processor), output filter 230 can pass filtered indicated flow 620to the output port of the DSP (or other micro-processor).

As can be noted from the example in FIG. 6, output filter 540 removesthe noise and transient spikes indicated flow 530. Because noise can beremoved from, or, at least, substantially reducing the indicated flow,embodiments of the present invention can create a filtered indicatedflow which is less likely to cause false alarm conditions inmanufacturing tools, thereby reducing downtime and wafer batch loss.Furthermore, this can be done without reducing the response time of themass flow meter.

Embodiments of the present invention provide output filters that canremove noise and transient spikes from an indicated flow, whilecontinuing to show persistent pressure changes in the indicated flow.The filtered indicated flow can be compared to a set point, and if thefiltered indicated flow deviates from the set point by a large enoughextent, an alarm can be generated, shutting down associatedmanufacturing tools. Because the filtered indicated flow contains lessnoise and fewer transient pressure spikes, it is much less likely tocause false alarms, thereby reducing manufacturing tool downtime andsaving considerable expense. Additionally, the embodiments of thepresent invention do not reduce the response time of mass flowcontrollers and mass flow meters.

Although the present invention has been described in detail herein withreference to the illustrative embodiments, it should be understood thatthe description is by way of example only and is not to be construed ina limiting sense. It is to be further understood, therefore, thatnumerous changes in the details of the embodiments of this invention andadditional embodiments of this invention will be apparent to, and may bemade by, persons of ordinary skill in the art having reference to thisdescription and practice of the invention disclosed herein. It iscontemplated that all such changes and additional embodiments are withinthe intent and true scope of this invention as claimed below.

1. A method for filtering an indicated flow comprising: filtering saidindicated flow to derive an intermediate filtered indicated flow;deriving a weighted first derivative of said intermediate filteredindicated flow; and summing said intermediate filtered indicated flowand said weighted first derivative of said intermediate filteredindicated flow to derive a filtered indicated flow; wherein saidintermediate filtered indicated flow is derived using running average.2. A method for filtering an indicated flow comprising: filtering saidindicated flow to derive an intermediate filtered indicated flow;deriving a weighted first derivative of said intermediate filteredindicated flow; and summing said intermediate filtered indicated flowand said weighted first derivative of said intermediate filteredindicated flow to derive a filtered indicated flow; wherein saidintermediate filtered indicated flow is derived using IIR.
 3. A methodfor filtering an indicated flow comprising: filtering said indicatedflow to derive an intermediate filtered indicated flow; deriving aweighted first derivative of said intermediate filtered indicated flow;and summing said intermediate filtered indicated flow and said weightedfirst derivative of said intermediate filtered indicated flow to derivea filtered indicated flow; wherein said intermediate filtered indicatedflow is derived using FIR.
 4. A system for filtering an indicated flowcomprising a flow device operable to: filter an indicated flow to derivean intermediate filtered indicated flow; derive a weighted firstderivative of said filtered indicated flow; and derive a filteredindicated flow from a sum of said intermediate filtered indicated flowand said weighted first derivative of said intermediate filteredindicated flow; wherein said intermediate filtered indicated flow isderived using running average.
 5. A system for filtering an indicatedflow comprising a flow device operable to: filter an indicated flow toderive an intermediate filtered indicated flow; derive a weighted firstderivative of said filtered indicated flow; and derive a filteredindicated flow from a sum of said intermediate filtered indicated flowand said weighted first derivative of said intermediate filteredindicated flow; wherein said intermediate filtered indicated flow isderived using IIR.
 6. A system for filtering an indicated flowcomprising a flow device operable to: filter an indicated flow to derivean intermediate filtered indicated flow; derive a weighted firstderivative of said filtered indicated flow; and derive a filteredindicated flow from a sum of said intermediate filtered indicated flowand said weighted first derivative of said intermediate filteredindicated flow; wherein said intermediate filtered indicated flow isderived using FIR.